Complicated variations in the early optical afterglow of GRB 090726

Aims. We report on the detection of an early rising phase of optical afterglow (OA) of a long GRB 090726. We resolve a complicated profile of the optical light curve. We also investigate the relation of the optical and X-ray emission of this event. Methods. We made use of the optical photometry of this OA obtained by the 0.5 m telescope of AI AS CR, supplemented by the data obtained by other observers, and the X-ray Swift/XRT data. Results. The optical emission peaked at ${\sim}17.5$ mag(R) at $t - T_{0} \approx 500$ s. We find a complex profile of the light curve during the early phase of this OA: an approximately power-law rise, a rapid transition to a plateau, a weak flare superimposed on the center of this plateau, and a slowly steepening early decline followed by a power-law decay. We discuss several possibilities for explaining the short flare on the flat top of the optical light curve at $t-T_{0} \approx 500$ s. Activity of the central engine is favored, although reverse shock cannot be ruled out. We show that power-law outflow with $\Theta_{\rm obs}/\Theta_{\rm c} > 2.5$ is the best case for the OA of GRB 090726. The initial Lorentz factor is $\Gamma_{0} \approx 230{-}530$ in the case of propagation of the blast wave in a homogeneous medium, while propagation of this wave in a wind environment gives $\Gamma_{0} \approx 80{-}300$. The value of $\Gamma_{0}$ in GRB 090726 thus falls into the lower half of the range observed in GRBs and it may even lie on the lower end. We also show that both the optical and X-ray emission decayed simultaneously and that the spectral profile from X-ray to the optical band did not vary. This is true for both the time periods before and after the break in the X-ray light curve. This break can be regarded as achromatic. The available data show that neither the dust nor the gaseous component of the circumburst medium underwent any evolution during the decay of this OA, that is, after $t-T_{0} < 3000$ s. We also show that this OA belongs to the least luminous ones in the phase of its power-law decay, corresponding to what is observed for the ensemble of OAs of long GRBs